Let’s know why was it introduced..

Bonding agents were introduced to improve or to create the bond between the tooth and the Restorative material.

Which was the first bonding agent ??

It’s sevriton cavity seal. It’s based on glycerophosphoric acid dimethacrylate. It had limitations like high polymerization shrinkage and high thermal expansion.

Composition of a dentin bonding agent:-

Primer + Etchants + adhesive , solvents, initiators, fillers particles and others ingredients like polyalkenoic copolymer

• Use of primer it’s to wet the surface properly or to reduce the contact angle. They maintain an expanded collagen network to allow filtration of hydrophobic monomer as well. Ex:- HEMA( Hydroxy ethyl methacrylate ) and TEGDMA ( Tri ethanol glycol dimethacrylate ).

• Conditioners are also called as Etchants. 10% Malic acid for 1 minute , and 37% phosphoric acid for 15 sec and if the concentration of phosphoric acid exceeds more than 50% then it forms monocalcium phosphate monohydrate which inhibits dissolution. Etching is done for both enamel and dentin.

• Mode of action of etchant on enamel :- by selective dissolution of enamel rods at the centre or peripheries or both. They act by increasing the surface energy and increases the surface area which creates microporosities on the surface of enamel.

• Mode of action of etchant on dentin :-It involves removal of smear layer and opening of dentinal tubules

Smear layer :- During tooth preparation the cut material along with water forms a thin film on the floor of the cavities known as smear or debris layer

Smear layer is it desirable? ?

In dentin the smear layer becomes burnished into the underlying dentinal tubules and it lowers the dentin permeability which has a protective effect.

Developing a bonding agent for enamel was easier than those of dentin, What would be the reason ?

Dentin contains water and due to inhomogeneous composition and presence smear layer will be a problem, whereas for enamel it contains more inorganic content that is calcium hydroxyapatite crystals.

• The purpose of using solvents is in order to increase the diffusion of primers and adhesive into micro retentive tooth surface and most common solvents used are ethanol and acetone.

• Adhesives in 1st generation are GPDM, 2nd generation BISGMA, TEGDMA, 3rd generation NPG- GMA,4th generation NPG-GMA, HEMA, 5th generation PENTA , Methacrylated phosphonates, 6th and 7th generation methacrylated phosphonates in water

• The etchant generally was 37% phosphoric acid almost for all generations; and primer was HEMA.

• Initial generations had all the components primer+ etchant + adhesive separately; as time progressed new generations evolved with all in one bonding agent.

• 2 step method :- 5th and 6th generation; 1 step method :- 7th generation ( all in one system )

• Indications for use of bonding agent:- When composite resin is used as Restorative material; And even when the porcelain veneers are bonded and when the exposed dentin is to be desensitized.

• Conditions that satisfy the true adhesion of Restorative material with tooth structure are sound tooth structure must be conserved; optimal retention must be achieved, microleakage should be prevented.

Hybrid layer :- Structure formed in dental hard tissues by deminerlization of surface followed by infiltration of monomers into collagen mesh and subsequent polymerization is called hybrid layer. It was reported by Nakabayshi in 1982.

Reference :- Philips and Manapalil text book of dental materials

Thermal properties

The arrangement of atoms and molecules in materials is influenced by the temperature; as a result, thermal techniques are important in understanding the properties of dental materials

Thermal conductivity : thermal conductivity of a substance is the quantity of heat in calories or joules per second passing through a body 1cm thick with a cross section of 1 sq.cm when the temperature difference is 1°C.

Eg – a large amalgam filling or gold crown in proximity to the pulp may cause the patient discomfort when hot or cold foods produce temperature changes, this effect is mitigated when adequate tooth tissue remains or cavity liners are placed between the tooth and filling for insulation.

Specific heat : specific heat of a substance is the quantity of heat needed to raise the temperature of 1g of the substance by 1°C.

Eg – during the melting and casting process, the specific heat of the metal or alloy is important because of the total amount of heat that must be applied to the mass to raise the temperature to the melting point.

Thermal diffusivity : it is a measure of transient heat flow and is defined as the thermal conductivity, divided by the product of the specific heat, times the density.

Eg- for a gold crown or a dental amalgam, the low specific heat combined with the high thermal conductivity creates a thermal shock more readily than normal tooth structure does.

Coefficient of thermal expansion : the change in length per unit length of a material for a 1°C change in temperature is called the linear coefficient of thermal expansion.

Although the coefficient is a material constant, it doesn’t remain constant over wide temperature ranges. For eg, the linear coefficient of thermal expansion of a dental wax may be an average value of 300×10-6/°C upto 40°C, whereas it may have an average value of 500×10-6/°C from 40-50°C.

The coefficient of thermal expansion of a polymer changes as the polymer goes from a glassy state to a softer, rubbery material. This change in the coefficient corresponds to the glass transition temperature.

It is obvious that with the reduction in temperature, there is a contraction of a substance as much as of expansion tht occured during heating. Accordingly, tooth structure and restortive materials expand when warmed by hot food or beverages and contract when exposed to cold substances. Such expansions and contractions may break the marginal seal of a filling in tooth particularly when the difference between coefficient of thermal expansion of tooth and restorative material is too large.

Electrical properties

The ability of a material to conduct an electric current may be stated as conductivity or conversely as the specific resistance or resistivity. The conductivity by materials used to replace tooth tissues is of concern in restorative dentistry.

Dielectric constant : a material that provides electrical insulation is known as dielectric. The dielectric constant of a dental cement generally decreases as the material hardens. This decrease reflects a change from a paste that is relatively ionic and polar to one that is less.

Electromotive force : the electromotive series is a listing of electrode potentials of metals according to the order of their decreasing tendency to oxidise in solution. Those metals with a large negative electrode potential are more resistant to tarnish than those with a high positive electrode potential.

Galvanism : the presence of metallic restorations in the mouth may cause a phenomenon called galvnic action where saliva or bone fluids like electrolytes make up an electric cell.

Corrosion : the corrosion of gamma, gamma 1, gamma 2 phases in amalgam has been studied by electrochemical analysis. The dental amalgam specimens become pitted at the boundaries between the phases or in gamma 2 phase. The addition of copper to amalgam alloys to form copper-tin compounds during hardening has improved the resistance of amalgam to chloride and galvanic corrosion.

Tarnish : the process of steam sterilization of surgical instruments has long presented a serious problem of tenish and corrosion. Many non metallic materials such as cements and composites have shown a tendency to discolor in service because the colored substances penetrate the materials and continue chemical reactions in the composites.

Source : Craig’s textbook of restorative materials